The link halemano posted is a terrific source of info!
FYI, conditions at the volcano change on a daily basis, so keep checking. Also check for any Fume Advisories since recently parts of the park (Chain of Craters Road) have been closed on & off due to heavier than usual volcanic fumes.
Even during times when the lava flow is viewable "on foot", you normally have to drive through the Hawai'i Volcanoes National Park to get to the trail head, which will mean you'll be above 3,000 feet for part of the drive, so keep that in mind as well if you plan to dive... but there is lots of cool stuff to see up in the National Park even if you do not get to hike to the lava!
Safe Diving!